Fix TypeScript errors, update Calendar component, and add Vitest

[google-labs-jules]

This PR addresses the issues found during the project check:

1. Bug Fixes: Resolved runtime and type errors in seed-db.ts caused by incorrect Firebase Modular SDK usage.
2. Component Update: Updated the Calendar component to be compatible with react-day-picker v9, fixing build errors.
3. Testing: Added vitest and a sample unit test to establish a testing foundation for the project.

---

PR created automatically by Jules for task 1880398995458110509 started by @ViVaLaDaniel

[google-labs-jules]

👋 Jules, reporting for duty! I'm here to lend a hand with this pull request.

When you start a review, I'll add a 👀 emoji to each comment to let you know I've read it. I'll focus on feedback directed at me and will do my best to stay out of conversations between you and other bots or reviewers to keep the noise down.

I'll push a commit with your requested changes shortly after. Please note there might be a delay between these steps, but rest assured I'm on the job!

For more direct control, you can switch me to Reactive Mode. When this mode is on, I will only act on comments where you specifically mention me with @jules. You can find this option in the Pull Request section of your global Jules UI settings. You can always switch back!

New to Jules? Learn more at jules.google/docs.

---

For security, I will only act on instructions from the user who triggered this task.

[coderabbitai]

Important

Review skipped

Bot user detected.

To trigger a single review, invoke the @coderabbitai review command.

You can disable this status message by setting the reviews.review_status to false in the CodeRabbit configuration file.

---

_{Comment @coderabbitai help to get the list of available commands and usage tips.}